The increasing use of optical fiber in optical communications systems has resulted in a demand for extremely large quantities of fiber. In the production of suitable fibers, a continuous process for drawing the fiber from a heated to softness glass preform at relatively high speeds, such as, for example, thirty-five meters per second, (35 m/sec.) is almost universally used. It is also the common practice that, during the draw process, a coating of, preferably, polymeric material, is applied to the fiber. This coating functions to protect the glass fiber from nicks, scratches, and other environmental concerns, and further, increases the structural strength of the fiber when so coated.
There are strict operational constraints that apply to glass fibers in use in, for example, an optical communication system, thus, the fiber manufacturing process is, customarily, closely monitored and controlled to eliminate defects in the fiber occurring during the draw process. The coating on the fiber, if faulty, can also have a deleterious effect on the fiber performance, hence the coating application process should be closely monitored also. Thus, monitoring of the coating is directed primarily to such parameters as diameter, elliptically, and concentricity, which are slow changing parameters and can be detected and evaluated over considerable fiber lengths. In general, devices employing various scanning techniques are utilized to monitor these slow changing parameters. However, the particular devices presently used to identify and measure these slow changing parameters are not capable of accurately identifying and responding to defects which are temporarily short in duration at line draw speeds and thus quite often such defects escape detection by present day monitors which employ scanning techniques. Such defects may be caused by inclusion of particulates, which may alter the coating diameter, entrapped bubbles, or high viscosity areas in the coating pulled through the coating application die or by the particular shape or configuration of the defect. Each of these defects, as well as others, may cause a loss of lightguide product during subsequent processing. The only such defects that existing devices consistently and accurately detect are those which happen to be exceptionally large.
In European patent EPO 553987 Al of Frazee, et al., of previously common assignee, the disclosure of which is incorporated by reference herein, there is shown and described a monitoring apparatus which applies one or more orthogonally intersecting light beams to a coated fiber. As the light passes through the fiber and coating, a refraction pattern having a predictable intensity level is produced by the forward scattered light beams. The intensity of the forward scattered pattern is continuously monitored by a plurality of photo-diodes. Various defects in the coating cause the light path through the coating to be altered, thereby directing some of the forward scattered light outside of the normal pattern into the detection area of the photo-diodes, producing an increase in the light intensity measured by these diodes, thus indicating the presence of forward scatter produced by short defects in the coating.